The object of the invention is an apparatus in the drying section of a paper machine, with which the detachment of paper from the smooth surface of a drying cylinder is improved. The dynamic forces caused by movement and the adhesive attraction between surfaces cause the paper in the cylinder drying section of a paper machine want to follow the surface of the smooth drying cylinder at the point where it is desired that the paper detaches from the surface of the drying cylinder and after that, supported by the next drying wire, to go to the next drying cylinder via a hitch roll. It is endeavored to solve this problem by installing apparatuses that produce low pressure on the side opposite the paper. Conventionally, a low pressure is formed for the whole distance that the paper travels on the surface of the drying wire from the drying cylinder to the hitch roll and to the next cylinder.
As speeds increase, dynamic forces also increase and for this reason there has been a need to raise the level of the low pressure on the backing side of the drying wire, so that the paper remains attached to the drying wire. For this reason one problem, among others, that arises is the bending of the drying wire, because it cannot be tightened too much owing to, in turn, the bending of the rolls.
Apparatuses have been developed to solve the problem, wherein a greater low pressure is formed in particular at the point of detachment of the paper from the cylinder.
The level of prior art generally is described in e.g. patent publications U.S. Pat. No. 5,782,009 and FI-110625B.
In the solution according to publication U.S. Pat. No. 5,782,009 different low pressure areas are formed, which areas are separated by mechanical seals. Low pressure is formed in the detachment area with a dedicated fan/suction apparatus, the area is isolated with mechanical seals from the rest of the area of lower pressure in which, correspondingly, a different low pressure is formed with a fan/suction apparatus dedicated to it. In this way different low pressures are achieved for the detachment area and for the rest of the area.
The solution to the problem is, however, in how the mechanical seals are brought close enough to the drying wire for the low pressure needed to be achieved. If the seals are left too far away, e.g. to prevent mechanical contact, a lot of air must be sucked out in order to achieve an adequately low pressure. Correspondingly a seal that is brought too close easily rubs against the drying wire and wears it until it is unusable.
In the solution according to publication FI-110625B the low-pressure areas are divided into areas of high low-pressure and low low-pressure. The area of high low-pressure is made by blowing air against the direction of travel of the drying wire, which ejects air along with it and forms low pressure in the space behind it. The enhanced low-pressure space is limited either with a mechanical seal pushed towards the drying wire or alternatively with a nozzle blowing in the opposite direction.
In a solution in which the area of greater low-pressure is limited with a mechanical sealing, one problem is adjusting the mechanical sealing to exactly the correct distance. Too great a distance does not produce the necessary pressure difference between the high low-pressure area and the low low-pressure area. Correspondingly, too small a distance causes bending of the wire and rubbing of the seal on the wire.
In an alternative solution, in which a separate nozzle blowing in the direction of travel is used, one problem that arises is the opposing air flows. In this case the sealing implemented with blowing in the direction of travel of the drying wire is subjected to turbulent air flows between the high low-pressure area and the low low-pressure area in the hitch roll zone.